This invention pertains to methods for producing capping layers for metal lines in integrated circuits. The use of diffusion barriers and capping layers to fully encapsulate copper lines within copper containing integrated circuits is an accepted strategy for limiting copper diffusion into surrounding insulating dielectric materials.
Generically, a diffusion barrier is a thin layer of material that is deposited at the interface between two materials to prevent diffusion from one material into the other. In a typical situation, the diffusion barrier exists at the interface between conductive and insulating layers in an integrated circuit and prevents metal diffusion into non-conductive areas. In modern ULSI processes, diffusion barrier materials line the trenches and vias provided in a dielectric layer during damascene processing. They are also used in “capping” layers that cover the “top” surfaces of deposited copper lines (i.e., the surfaces of the copper lines that are coplanar with the dielectric field regions).
Diffusion barrier materials should have low resistivity and adhere well to both dielectric material and metal (e.g. copper). Conventionally, diffusion barriers for copper comprise refractory metals and compounds of refractory metals since these metals have the requisite low resistivity, adhere well to copper and can maintain good barrier properties at high temperatures. Titanium, tantalum and tungsten metals and their nitrides are commonly used. More recently, other refractory metal compounds such as nickel, molybdenum, and cobalt have been investigated.
In addition to serving as a diffusion barrier, the capping layer can help mitigate the problem of electromigration. Electromigration is a material transport phenomenon resulting from the drift or movement of atoms under the influence of momentum imparted on the atoms of the metal lattice by electron scattering (sometime viewed as an “electron wind”) that is created by very high current densities flowing in small wires. In electromigration, micro-voids form between the current-carrying medium (e.g., copper of the copper line) and the surrounding barrier or insulator as the metal atoms move with the flowing electrons (“downstream”) the micro-voids move in the opposite direction (“upstream”). This ultimately creates areas of less metal in cross section and even relatively large voids in copper lines at, for example, regions near a via base. Electromigration micro-voids are most pronounced at interfaces with the copper line where adhesion is weakest. Because cobalt and some related transition metal elements provide particularly good adhesion to copper, these materials have been proposed and are being gradually accepted as capping layers in the industry. The alloying character of the metal is also believed to contribute to the reduction of the electromigration behavior, binding the metal at microscopic void areas such as grain boundaries.